Generally, in an image forming apparatus of electrophotographic recording system, photoconductive drums are arranged in a line and electrostatic latent images are formed by applying laser beams to the respective photoconductive drums. The photoconductive drums form toner images of respective colors with developing apparatuses and a color image is obtained by multiply-transferring the toner images of respective colors on a sheet of paper.
The developing apparatuses are provided for the respective photoconductive drums, toner cartridges are provided for supplying toner to the developing apparatuses, and the toners contained in the toner cartridges are carried to the developing apparatuses.
The developing apparatus has a developing roller for moving the toner to the photoconductive drum and a mixer for stirring a developer. Within the mixer, a stirring roller that stirs the toner and carrier is provided, and the developing roller and the stirring roller are simultaneously operated using the same drive source (motor).
In the image forming apparatus, printing processing is performed at some process speeds. Assuming that printing on paper having a normal thickness is processed at the normal speed, for example, printing on thick paper or special paper is processed at a reduced process speed to ½ speed, ⅓ speed, or the like.
For the electrophotographic recording system having some process speeds, if the developing roller and the stirring roller are driven by the same drive source and the rotational ratio between the developing roller and the stirring roller is constant, the following defects are caused.
The image forming apparatus has some operation modes, for example, modes for executing a printing job at the normal speed, a printing job at the ½ speed, and a printing job at the ⅓ speed. However, if the developing roller and the stirring roller rotate at the same rotational ratio, when the printing job at the ½ speed and the printing job at the ⅓ speed are executed, the rotational speed of the developing roller becomes lower, the stirring roller also rotates at the lower speed, and thereby, stirring of toner becomes insufficient. Accordingly, the next printing is started with the insufficient charge and the inhomogeneous concentration ratio of toner to carrier, and causes density irregularities in image.
Further, the apparatus also has a forcible toner supply mode and an automatic toner adjustment mode other than the modes of executing printing jobs at the ½ and ⅓ speeds. The forcible toner supply mode is a mode of supplying and stirring toner when the toner concentration becomes lower to less than a predetermined level. The automatic toner adjustment mode is a mode of automatically stirring toner and carrier to adjust the toner concentration when the power is on.
It is unnecessary to rotate the developing roller in the forcible toner supply mode and the automatic toner adjustment mode because printing on paper is not performed, however, the developing roller is arranged to rotate following the stirring roller and the developing roller idly rotates.
Japanese Patent No. 3774123 discloses an image forming apparatus in which a developing roller and a stirring roller are independently driven and, when the power is on, the developing roller is stopped and only the stirring roller is driven.
Further, JP-A-2006-154193 discloses a developing apparatus in which a developing roller and a stirring roller are independently driven and their numbers of rotations are switchable.
Furthermore, JP-A-2005-24822 discloses an image forming apparatus in which numbers of rotations of a developing roller and a stirring roller are individually controllable.
However, the conventional image forming apparatus is not able to control the rotations of the developing roller and the stirring roller according to some process speeds, and the developing roller idly rotates, and the toner may fly and the developer may be degraded. Further, defects such that the stirring by the stirring roller is insufficient are caused.